Recently a semiconductor tends to become high integration and high performance such as Very Large Scale Integration (VLSI) or Ultra Large Scale Integration (ULSI). Therefore a design rule of wiring pattern also tends to become fine such as submicron or quartermicron. Aluminum (Al) has been used as a wiring material in a semiconductor device. In this specification, aluminum (Al) means pure aluminum or a material, mainly including Aluminum. But aluminum wiring is easy to deteriorate a reliability owing to a deterioration of an electro-migration durability and/or a stress-migration durability when a wiring width is smaller than 0.5 .mu.m. Further a layer of a wiring becomes thick in order to get a low resistance. It makes an aspect ratio of a wiring pattern large such as from 1 to 2. It makes difficult to flat by a insulation layer in the latter process.
Thus because aluminum wiring has some problems as stated above, copper (Cu) wiring is used preferably. In this specification, copper (Cu) means pure copper or a material mainly including copper. Copper has a high electro-migration durability. The electric resistivity of pure copper is about 1.4 .mu..OMEGA.cm. It is about a half of the electric resistivity of pure aluminum. Therefore even though a wiring pattern of copper has the same width as a wiring pattern of aluminum has, a thickness of a copper film becomes thinner than a thickness of a aluminum film. It is advantageous to form a multi-film wiring.
But there are some problems to etch copper in dry process. The research for fine processing of copper is on the way.
Copper is very easy to be oxidized. A surface of copper is always covered by copper oxide (CuO) and a passive state. When copper oxide exists on a surface of copper, copper oxide obstructs a reaction between an etching gas and copper. It declines an etching velocity. And because copper oxide acts on copper as a mask, a spot of copper and/or copper oxide on a wafer is remained after an etching.
Further it is difficult for chloric gas used as an etching gas of aluminum to etch copper, because a vapor pressure of cuprous chloride being a reaction product is very low.
Some ideas have been suggested in order to solve these problems until now.
For instance, Japanese Journal of Applied Physics., Vol. 28, No. 6, p. L1070-1072 (1989) shows a reactive ion etching that a copper thin film is etched by a mixed gas of silicon tetrachloride (SiCl4) and Nitrogen (N2) while heating a wafer at the approximate temprature of 250.degree. C. This heating prevents a spot of copper and/or copper oxide from remaining on a wafer after an etching. The reactive product silicon nitride (SixNy) in a gaseous phase is utilized as a side wall protector, and anisotropic processing is performed.
Japanese laid open patent Hei 1-234578 shows that hydrogen (H2) of some percentages is added into an etching gas. Though copper oxide is produced at a surface of copper by a reaction with residual oxide at a high temperature, copper oxide is reduced by H2 added into an etching gas. Therefore copper is smoothly etched in dry process.
Further Japanese laid open patent Hei 3-295232 shows that a mixed gas of nitrogen (N2) and oxygen (O2), a gas having an oxygen atom and a nitrogen atom in a molecule and/or fluorine gas is added into an etching gas while heating a wafer at the temperature of less than 200.degree. C. Copper is sublimated in the form of copper nitrate ( Cu(NO.sub.3).sub.2 ) and removed in this process.
However the ideas stated above have some problems which must be solved.
At first, silicon nitride (SixNy) may be possible to be a particle contamination source when silicon nitride (SixNy) is utilized as a side wall protector.
Further when silicon nitride (SixNy) is utilized as a side wall protector or the copper oxide is reduced by H2 added into an etching gas, it is necessary to heat a wafer at a high temperature. It is impossible to use a resist mask as a etching mask at the high temperature. So heat-resisting material such as silicon dioxide (SiO.sub.2) is utilized as an etching mask. But it is difficult to remove the mask of silicon dioxide after etching because there is few solvent to have a selectivity between silicon dioxide and a interlaminar insulating film. If the mask of silicon dioxide is utilized as a part of a interlaminar insulating film without removing of the mask, surface steps of the wafer increase. A forming of an etching mask is an extra process itself. Further a heating of a wafer at a high temperature accelerates forming of a copper oxide film by residual oxygen in a chamber.
Meanwhile in the method of removing copper as copper nitrate, there is no active material of halogen in an etching reaction system. It means that a selective rate between a mask pattern and a interlaminar insulating film becomes large. Further it is advantageous that a resist mask is utilized as a etching mask, because a wafer is heated at a low temperature. But because an etching gas includes oxygen, a copper oxide film is formed on a surface of copper. An oxygen atom diffuses inside at the place formed a copper oxide film. It causes problems that a spot of copper and/or copper oxide remains on a wafer after an etching and a wiring resistance increases.